This invention relates to wallcovering product that comprises a decorative sheet material which carries a layer of pressure sensitive adhesive and which is supplied in the form of a roll.
Conventional wallcoverings either require wet adhesive to be applied to the back of the sheet material for application to a wall or, in the case of ready pasted products, require application of water. These products are inconvenient to use, since the handling of the paste and brushes or water troughs requires that furniture and preferably also carpets be removed from the vicinity of the walls to which the wallcoverings are being applied.
In addition, wet wallcoverings become soft and present handling difficulties. Stretching of the product can occur and can lead to difficulties in matching the pattern repeat on the walls. Walls with lining paper must be sized with glue or the lining paper must be removed before application of further wallcovering and these tasks are inconvenient and time consuming. Once these types of products are applied to the wall and the adhesive has dried it is, however, rare for the dimensional stability, or rather inherent lack thereof, of paper-based substrates to cause problems on changing humidity.
In order to overcome the above-mentioned problems, it has been proposed to provide wallcovering material with a pressure sensitive adhesive or a heat activatable adhesive already in place so that a roll can be unrolled and immediately applied to a wall surface without wetting being necessary. However, generally wallcoverings comprise cellulosic substrates primarily for reasons of economy and there are difficulties with the dimensional stability of these types of products when in place on a wall due to changing moisture content of the atmosphere, especially when exposed to humid environments such as bathrooms. Dimensional stability is a particular problem when the sheet material comprises an adhesive layer, a substrate layer and a decorative-surface layer, the surface layer being provided on a layer of polymeric material such as for example, PVC, ie, a vinyl wallcovering. This type of wallcovering is described for example, in GB 1264795 and GB 1315114.
The particular difficulty arises with polymeric coated substrates, when the atmosphere becomes humid, because water vapour is transmitted through the polymeric coating into the substrate layer. Although this might not be a problem, if the substrate is wholly non-hydrophilic, for economic reasons it always contains hydrophilic material. As a result the transmitted moisture from the atmosphere will be absorbed by the substrate layer and this will cause expansion which may be considerable. This can then give rise to bubbling of the product on the wall or the edges of adjoining strips can become raised. Also, when the external air temperature rises and relative humidity falls, moisture escapes from the product and can cause the product to shrink due to the fibre shrinkage and so leave gaps between strips, though this is less of a problem than that of expansion.
Various methods have been suggested to overcome the problems of dimensional stability of this type of wallcovering product. One way to overcome the problem of dimensional stability could be to use a substrate which does not include hydrophilic material so that differing humidities will not affect the material. Suitable non-hydrophilic substrates include for example glass fibre and polyester fibres. However, such substrates are very expensive and therefore not preferred.
In DE-A-3741194, in order to avoid formation of creases in the wallcovering, it is suggested to use a substrate which expands relatively little in damp conditions, in addition to using an adhesive in the form of small islands of foam so that expansion and contraction which give dimension changes in the substrate caused by moisture changes in the room, will be diverted in a direction away from the wall surface due to micro expansion between adhesive points. In GB 2117271, "dry-on dry-off" wallcoverings, i.e. those which do not require a wetting stage for their application and which use a pressure-sensitive or heat activated adhesive are described. The problem of dimensional stability of paper-based substrates with increasing as well as lowering humidity is said to be overcome in that disclosure by using dimensionally stable substrate material such as a closed cell foamed film of LDPE, ethylene copolymer non-woven or spun-bonded products, including a glass fibre stabilised cellulosic web. Whilst these products may give greater dimensional stability than conventional cellulose substrates, they tend to be extremely expensive and still do not give total dimensional stability. The cellulose containing substrate will still absorb water in humid atmospheres and expansion and contraction of the product will therefore result.
GB 1241177 also recognises that changing moisture content can affect the dimensional stability of paper-based substrates and that pre-pasted coatings which are activated by moistening cause particular dimensional instability problems. The solution is to use adhesives activatable by heat and/or pressure rather than by application of water. Wetting the paper prior to application is suggested to ensure against buckling when the paper is to be hung at high humidities. However, this counteracts the benefit of having a pressure-sensitive adhesive already in place, if a wetting step is still necessary for application. In addition, such a wetting step produces the problems discussed above in that wet wallcoverings become soft and present handling difficulties and stretching may occur.
In addition to the dimensional problems of self adhesive coated wallcoverings, caused by varying humidity, generally, the adhesive is applied in an aqueous or other solvent-based composition and excess water is subsequently removed in a drying step. Typically when the adhesive is aqueous based, it will contain from 40 to 70% water or organic solvent at the point of application to the web and the drying temperatures vary considerably but are generally in the range of 50.degree. to 150.degree.0 C. or more. As a result the substrate shrinks and is usually dried to below the moisture content it would have after prolonged exposure for a normal atmosphere. If it is applied to the wall in this shrunk state, normal atmospheric humidity is liable to cause expansion.
In practice the wallcovering is packed in a roll that is either freely exposed to the atmosphere or is enclosed in a plastic wrapping that is usually open at the ends. Some equilibration with atmospheric humidity will normally occur, but the effect may be non uniform with the result that the wallcovering will have varying moisture content, and stretch potential, along its length.
We have now found that problems resulting from the lack of dimensional stability in use in the normal range of environments can be overcome if the wallcovering is supplied ready for application to the wall at particular dimensions such that the expansion possible under usual conditions of use, beyond the original dimensions, is so small as not to cause bubbling or raising of edges, this being achieved preferably by ensuring that the moisture content of the material in the roll is greater than the equilibrium moisture content under normal conditions.